The present invention relates to a method of fabricating a dynamic random access memory for a semiconductor integrated circuit and, more particularly, to a method of fabricating a capacitor of a memory cell having a structure in which the capacitor is formed after a bit line is formed.
Conventionally, a memory cell of this type is constituted by a single MOS transistor and a capacitor having a stacked structure which is composed of a silicon oxide film and a silicon nitride film and has polysilicon as an electrode. A technique for forming this capacitor after formation of a bit line is described in, e g., "International Electron Devices Meeting Digest of Technical Papers," 1988, PP. 592-595. In this technique, after polysilicon serving as a lower electrode is formed, formation of a silicon dioxide film and a silicon nitride film is performed by a thermal oxidation method and a CVD (Chemical Vapor Deposition) method, respectively. Thereafter, polysilicon serving as an upper electrode is stacked, thus completing the capacitor.
In order for the above conventional memory cell to meet higher degrees of integration of recent integrated circuits, however, a reduction in the area of the capacitor must be compensated by decreasing the film thickness of a dielectric film and by effectively increasing the area by means of a three-dimensional structure, in which the capacitor is formed using the upper and side surfaces of the lower electrode. Since the dielectric film for forming the conventional capacitor consists of a silicon oxide film and a silicon nitride film, its dielectric constant is at most 7. Therefore, an extremely small film thickness (silicon oxide film) of 10 nm or less is necessary to realize the capacitor required. However, it is very difficult to form a dielectric thin film layer having a current-voltage characteristic lower than a permitted leakage current. In addition, in the method in which the electrode area is effectively increased by the three-dimensional structure, a leakage current is increased because the film thickness of a silicon oxide film is decreased or electric fields are concentrated at the end of the lower electrode. Furthermore, since the physical size of steps is increased by the three-dimensional structure, it becomes more difficult to form wiring without disconnections after the formation of the capacitor. This results in a reduction in yield.